1. Field of the Invention.
This invention relates generally to a total internal reflection (TIR) coupler for side coupling pump light into an optical fiber.
2. Description of the Related Art
Fiber amplifiers and fiber lasers require high optical pump levels to be injected within the region of the fiber that includes the active medium that provides the optical gain. Presently, the output power from fiber amplifiers and fiber lasers is limited by the amount of optical power that can be injected into the active medium of the fiber. This limitation stems from the approaches that have been used in the past to couple optical pump power into the fiber.
In the case of single-mode doped fiber, wavelength division multiplexing (WDM) couplers are typically used. The WDM coupler is connected between the telecom fiber and the single-mode doped fiber. Such an interruption complicates the pumping process.
In the case of dual clad doped fiber, a common approach for injecting optical pump power is end pumping. End pumping provides at most only two input ends through which all the injected optical pump power enters the fiber. This physically constrains the number and type of pump sources that can be used to inject the optical power. Such a constraint has led to the development of optical side pumping techniques for dual clad doped fiber in which the entire length of the fiber is made available to couple in a much greater number and type of optical pump sources.
Accordingly, side coupling pump light into a doped fiber has been performed in the past. As previously described, the use of WDM allows pump light having a different wavelength from that which is to be amplified to be coupled into the doped fiber at any point along the fiber provided that an interruption is made for the WDM coupler in the doped fiber. The WDM coupler is designed so that pump light is sent into one of the two input fibers and the light to be amplified is sent into the other input fiber. The interaction length between the two closely coupled fibers is precisely selected to enable the pump light to be completely coupled to the fiber with the light to be amplified while the light to be amplified remains in the same fiber. This is relatively complex.
U.S. Pat. No. 5,854,865 to Goldberg teaches that a second method for side coupling pump light is to cut V-grooves into the cladding of dual clad doped fiber. The V-grooves bounce the side-coupled light into a mode of propagation along the fiber. Although quite efficient ( greater than 90%), the cutting of the V-grooves, however, generally weakens the fiber, and is a complicated and expensive process.
Prism couplers are used to couple light into a dual clad doped fiber. The most common technique is to direct pump light into a prism positioned next to the cladding and rely on the evanescent field properties to couple light into the cladding. To achieve coupling efficiencies of 50-60% the spacing between the prism and the cladding must be precisely controlled. Representative evanescent prism coupling schemes are disclosed in U.S. Pat. Nos. 3,610,727 and 5,119,452.
Weber et al. xe2x80x9cA longitudinal and side-pumped single transverse mode double-clad fiber laser with a special silicone coatingxe2x80x9d Optics Communications 115 (1995) pp. 99-104 teaches placing the prism on a special silicone coating around a conventional circular clad fiber. Simple refraction is used to couple light into the prism and then into the fiber cladding. In this configuration, the angle of incidence at the air-prism interface is relatively large. An AR coating would have to be a multi-layer coating, which reduces transmission and increases cost. Operation at the Brewster angle eliminates the need for the AR coating but necessitates the use of a single transverse mode pump laser to get high transmission efficiency. Further, using simple refraction the angle of the pump light with respect to the fiber is relatively shallow. Therefore, to clear the path for the fiber, the pump and focusing lens must be relatively far away from the prism and the diameter of the focusing lens must be small. This also dictates the use of a single mode pump laser such as the Ti: Sapphire laser described in their paper.
U.S. Pat. No. 6,243,515 to Heflinger discloses a technique to side-couple light using a grating formed on a multimode fiber. The grating diffracts a beam of pump light at an angle that is matched to the angle characterizing a propagating mode. As shown in FIG. 7 of Heflinger""s patent, the diffracted beam can be first passed to a tapered fusion coupler, which directs the beam into the dual-clad fiber. The inclusion of a grating provides sub-optimal coupling efficiency and can be difficult to manufacture.
The industry has an unfulfilled need for a coupler, which is simple to construct, inexpensive, mechanically robust and provides coupling efficiencies even better than those provided by Goldberg""s V-groove.
In view of the above problems, the present invention provides a robust low-cost technique for efficiently side-coupling pump light into an optical fiber, which is particularly applicable to ultra-short optical amplifiers and lasers using multi-component glasses.
This is accomplished by bonding a TIR coupler in optical contact to the fiber""s inner cladding. The inner cladding, which surrounds a core, has at least one flat surface and is transparent to the pump wavelength. A source of pump radiation directs a beam of pump light into the TIR coupler from either the front or backside of the fiber, which is mounted on a substrate. The beam is preferably oriented substantially normal to the fiber to simplify packaging, facilitate the use of a multi-mode pump source and simplify the design of any anti-reflection (AR) coatings.
The TIR coupler has an angle of taper a and a length L such the principal ray of the pump light is reflected at an angle that satisfies the total internal reflection (TIR) condition at the coupler""s reflecting surface, and input and output coupling conditions, to efficiently xe2x80x9cfoldxe2x80x9d the light into the fiber and satisfies the TIR condition inside the fiber to xe2x80x9cguidexe2x80x9d the light down the fiber""s inner cladding. The angle of incidence is preferably such that substantially all of the pump light (principal and marginal rays) satisfies the TIR condition. The pump light is preferably focused to obtain such high coupling efficiencies and to confine the light within a narrow cladding, which produces higher power density.
Once coupled into the fiber, the pump light excites an active media thereby producing gain to either amplify an optical signal propagating through the core or satisfy the lasing condition to create an optical signal in the core. In one embodiment, the coupler directs pump light into the inner cladding surrounding an active media in the core, e.g. a coupler mounted on an active fiber. In another embodiment, the coupler directs pump light into a passive (undoped) section of the fiber, which in turn couples the pump light into the inner cladding surrounding the active media, e.g. a coupler mounted on a passive fiber that is optically coupled to an active fiber. This latter configuration provides significant advantages in devices that require ultra-short active gain medias such as amplifier chips or single-mode lasers and specifically those that use multi-component glasses highly doped with rare-earth ions to provide high gain per unit length.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: